For many years it has been known that a higher sulfide concentration in white liquor for kraft pulping in the pulp and paper industry (i.e. higher white liquor sulfur content) could produce kraft pulp with higher viscosity and better physical properties. It has also been known that the beneficial effect of higher white liquor sulfur content is more profound in the early stages of the kraft cook than in the later stages. However in the past there have been a number of technical and practical concerns and conditions which have precluded practical utilization of these concepts.
According to the present invention, it has been found that by treating the black liquor from the kraft pulping process in a particular way, it is possible to make two (or more) different sulfur content white liquor streams in a practical manner, and to use those plural streams in the production of kraft pulp. The manner in which the streams are produced, the source of sulfur for the streams, and the manner in which the streams are utilized are all distinct from earlier practices in the art, and result in significant advantages in the pulping process.
According to the invention, at least two different streams of white liquor having vastly different sulfur content are produced. According to the preferred method, the stream with the higher sulfur content is used to treat the slurry of comminuted cellulose (e.g. wood chips) used for the production of pulp first (e.g. in an impregnation stage), and then the stream with lower sulfur content, without lowering pulping selectivity. This allows one to improve sulfate pulping without changing the total overall sulfidity (that is without required significant additional sulfide, apart from what is already in the recirculated and recycled liquids and other substances) of the mill (i.e. without changing the overall sulfur balance).
Under some circumstances, according to the invention it is possible to treat the comminuted material with the low sulfur content white liquor first, and then the high sulfur content material, but this is not normally preferred.
The desired results according to the invention can be achieved in a practical manner by pressure heating the black liquor withdrawn from a continuous or batch kraft process, at a temperature and for a time period such that substantial volumes of off gases containing organic sulfur compounds are produced; treating the black liquor by incineration and recaustization to recover chemicals from them and utilizing the chemicals in the manufacture of the low sulfur stream of white liquor; treating the off gases to produce primarily hydrogen sulfide and methane; and absorbing the hydrogen sulfide into a stream of cooking liquor to produce the high sulfur content stream of cooking liquor (e.g. white liquor).
According to the most specific aspects of the present invention, the concepts described above are utilized in a practical manner to provide kraft pulp that has an optimum viscosity/kappa ratio, and/or good strength properties, and good yield. The invention is applicable to a wide variety of different kraft cooking processes, both batch and continuous. For example, the invention can be utilized in continuous digesters having an impregnation zone, or in cooking systems utilizing a separate impregnation vessel from the continuous digester, or in batch digesters. The invention can also be utilized in association with co-current movements of white liquor (or other cooking liquor) with the pulp, or countercurrent movements, such as are provided in the commercial digesting process marketed by Kamyr, Inc. of Glens Falls, N.Y. under the trademarks "MCC" and "EMCC".
White liquor is that treatment liquid in a kraft process which contains the active cooking chemicals, sodium hydroxide (NaOH) and sodium sulfide (Na.sub.2 S); or white liquor in an extreme situation may contain substantially only NaOH as the active ingredient; i.e. the term "white liquor" as used in the present specification and claims includes fresh sodium hydroxide. According to standard practice, the white liquor cooking sulfidity is maintained in the range of 25-45% (this overall sulfur content ratio is the amount of sodium sulfide to the sum of sodium sulfide and sodium hydroxide, expressed as sodium oxide, applied to the whole cooking process), and according to standard practice in the art, a sulfur content level below 15% results in a deterioration in the cooking rate and pulp quality. During the production of kraft pulp, residual or "black liquor" containing the reaction products of lignin solublization is withdrawn from the digester, concentrated, and burned in a recovery furnace to yield an inorganic smelt of sodium carbonate and sodium sulfide. The smelt is dissolved to form green liquor which is reacted with quick lime (CaO) to convert the sodium carbonate into sodium hydroxide and thereby regenerate the original white liquor.
According to the present invention, it has been found that by making two (or more) supplies or streams of white liquor, with different sulfur contents, and applying them to different portions in the kraft cycle, significantly improved results can be achieved. In fact it has been found that it is highly desirable to add the majority of the sulfide in the total white liquor (total cooking chemical) --at least about 60-90%, in fact even almost all the sulfide--at the initial stages (e.g. impregnation) of the kraft cycle, while adding white liquor with primarily only sodium hydroxide at the later stages (e.g. co-current or countercurrent cooks). Practicing the invention, it is possible to optimize the viscosity/kappa ratio of the produced pulp for any given level of sulfur content, and/or to optimize pulp strength properties. The increase can be dramatic, easily on the order of 25% for conventional cooks with multiple white liquor additions (co-current), about the same increase for modified continuous cooking (MCC.TM.), and 15% or more for extended modified continuous cooking (EMCC.TM.), without a significant change in the mill sulfur balance (i.e. amount of total sulfur utilized). Comparable increases occur for batch treatment too.
One aspect of the present invention is a method of producing chemical pulp: (a) Intentionally removing sulfur from a fluid containing sulfur. (b) Including by using removed sulfur from step (a), making at least two streams of white liquor, including a second stream having relatively low sulfur content, and a first stream having relatively high sulfur content, substantially higher than the sulfur content of the second stream. (c) Chemical pulping cellulose material utilizing the first and second streams of white liquor, added at different places in the pulping process. And, (d) washing the chemical pulp produced.
According to another specific aspect of the present invention there is provided a method of producing kraft pulp comprising the following steps: (a) Treating a slurry of cellulosic fibrous material in a plurality of consecutive stages with white liquor, including at least first and second stages, using a first supply of white liquor to the one stage and a second supply to another stage. (b) Subjecting the material treated with white liquor to cooking conditions in at least one stage of the stages to effect kraft cooking thereof, and producing black liquor. (c) Producing the second supply of white liquor, and separately recovering sulfur from the black liquor. And, (d) adding the recovered sulfur to the first supply of white liquor to significantly increase the sulfur content thereof so that it is greater than the sulfur content of the second supply. The first supply is preferably added to the first stage, and the second supply to the second stage.
According to still another specific aspect of the present invention there is provided a method of producing kraft pulp comprising the steps of: (a) Treating cellulosic comminuted fibrous material in a first stage with a first supply of white liquor. (b) Treating the material in a second stage with a second supply of white liquor. And, (c) controlling the relative sulfur contents of the first and second supplies of white liquor so as to optimize the viscosity/kappa ratio and/or physical strength properties of the kraft pulp produced.
While the invention will primarily be described in this application with respect to kraft cooking, there is a broader aspect of the present invention. According to the broadest aspect of the present invention, any lignin reaction enhancing or carbohydrate degradation slowing chemicals are targeted to one or more specific parts of the cook, such as the impregnation, co-current, or countercurrent stages. Thus the invention is applicable not just to kraft white liquor, but to sulfite process cooking liquors (containing a cationic base), anthraquinone, derivatives of or equivalents to anthraquinone, or the like. The anthraquinone may replace all or some of the sodium sulfide in a cooking liquor containing NaOH, or may be used in other liquor compositions, and polysulfide may also be utilized.
According to the broadest aspect of the present invention, a method of treating comminuted cellulosic fibrous material with cooking liquor to obtain cellulosic pulp, is provided. The method comprises the following steps: (a) Providing at least first and second cooking liquors, including a first cooking liquor having a first concentration of lignin reaction enhancing or carbohydrate degradation slowing chemical, and a second cooking liquor having a second concentration of that lignin reaction enhancing or carbohydrate degradation slowing chemical, the second concentration being from zero to much less than the first concentration. (b) Cooking the comminuted cellulosic fibrous material in several cook stages. (c) Targeting the first cooking liquor to one or more, but less than all, of the cook stages, to provide cooking liquor for one or more stages; and (d) utilizing the second cooking liquor in at least one cook stage in which the first cooking liquor is not utilized.
The cooking liquor utilized above may be a kraft cooking liquor, sulfite cooking liquor, or the like. It may contain a cationic base, sodium sulfide, anthraquinone or its derivatives or equivalents, or the like, and the first liquor may be targeted to an impregnation stage, co-current cook stage, or countercurrent cook stage.
It is the primary object of the present invention to provide a substantially improved cook cycle for the production of paper pulp, and a particular way of obtaining sulfur recovery for the production of different sulfur content white liquor streams in a practical manner. This and other objects of the invention will become clear from an inspection of the detailed description of the invention, and from the appended claims.